A genome-inspired DNA ligand for the affinity capture of insulin and insulin-like growth factor-2

The insulin-linked polymorphic region (ILPR) of the human insulin gene contains tandem repeats of similar G-rich sequences, some of which form intramolecular G-quadruplex structures in vitro. Previous work showed affinity binding of insulin to an intramolecular G-quadruplex formed by ILPR variant a. Here, we report on interactions of insulin and the highly homologous insulin-like growth factor-2 (IGF-2) with ILPR variants a, h, and i. Circular dichroism indicated intramolecular G-quadruplex formation for variants a and h. Affinity MALDI MS and surface plasmon resonance were used to compare protein capture and binding strengths. Insulin and IGF-2 exhibited high binding affinity for variants a and h but not i, indicating the involvement of intramolecular G-quadruplexes. Interaction between insulin and variant a was unique in the appearance of two binding interactions with K(D) approximately 10(-13) M and K(D) approximately 10(-7) M, which was not observed for insulin with variant h (K(D) approximately 10(-8) M) or IGF-2 with either variant (K(D)s approximately 10(-9) M). The results provide a basis for the design of DNA binding ligands for insulin and IGF-2 and support a new approach to discovery of DNA affinity binding ligands based on genome-inspired sequences rather than the traditional combinatorial selection route to aptamer discovery.